In the manufacture of color television receivers, integrated circuits are being used in increasing numbers for performing many of the complex circuit operations of the receivers. The use of such integrated circuits reduces the manufacturing cost of the receivers by substantially reducing the complexity of the manufacture.
One of the circuits of a color television receiver which has been successfully reduced to integrated circuit form is the color demodulator section of the receiver. This is the portion of the television receiver which is used to separate the color signals present in the NTSC color television signal. This signal includes a wideband brightness or luminance (Y) signal, synchronizing burst components, and a modulated chrominance subcarrier signal of approximately 3.58 megahertz. The subcarrier signal is phase and amplitude modulated by color difference signals (R-Y, B-Y, and G-Y), so that different phases of the subcarrier represent different hues of an image portion and the subcarrier amplitude relative to brightness (Y) at each phase represents the saturation of that hue. A monochrome or black and white receiver visibly reproduces only the Y component.
An integrated circuit color demodulator which has been successfully used in a large number of television receivers is disclosed in the U.S. Pat. No. to Cecchin and Hilbert 3,558,810. The demodulator of that patent includes three synchronous gated full-wave demodulators for producing color representative video signals for the red, green and blue guns or cathodes of a color television cathode ray tube. The system of the Cecchin/Hilbert patent is capable of operation as a "direct" demodulator in which the brightness components are applied to the demodulator sections in a push-push or balanced relationship, while the modulated subcarrier signal is applied in push-pull or opposite phases to the demodulator sections. This results in direct demodulation of the color representative signals without the need for any additional matrixing of brightness and color difference signal components.
The demodulator of the patent also is capable of operation as a color difference demodulator if the brightness signals are not applied to it. If this is done, it is necessary to add the brightness signal components to the demodulated color difference signals at some other part of the receiver.
The three demodulator sections of the Cecchin/Hilbert U.S. Pat. No. 3,558,810 are operated with only two reference signals applied to the integrated circuit demodulator from a reference oscillator section. The two signals chosen are representative of the red and blue color difference phases (R-Y and B-Y), and B-Y), and these are applied directly to the red and blue demodulator sections of the integrated circuit. In addition, a resistor adding circuit is used to combine a portion of the red color reference signal with a portion of the blue color reference signal to derive the switching signal applied to the green demodulator section to cause operation of that section at the proper phase.
Each section of a synchronous demodulator requires some predetermined minimum level of the switching signal for proper operation of the demodulator. In the Cecchin/Hilbert demodulator U.S. Pat. No. 3,558,810, the magnitude of the switching signals supplied by the color oscillator must be greater than this minimum amount for demodulation in the red and blue sections in order to supply a sufficient signal to the green section, because of the reduced amplitude of the two color reference input signals which are combined through the resistive adder to form the green demodulator switching signal. It is desirable to effect demodulation in three synchronous demodulators of the type shown in the Cecchin/Hilbert patent, but without requiring the higher level magnitude of the two color reference signals supplied to the demodulator section.